1. Field of the Invention
This invention relates to the field of disk drive storage devices. More particularly, this invention relates to a corrosion resistant electrical interconnect for a disk drive suspension and a method of producing such an electrical interconnect.
2. Description of Related Art
A disk drive generally uses several spinning storage medium (e.g. disks) to store data. Several read/write heads are positioned in close proximity to the disks by suspension assemblies. In a hard disk drive, a suspension assembly commonly refers to the combination of a slider/head, containing the read-write circuitry affixed to the distal end of the suspension. The suspension supports the slider as well as an electrical interconnect. The electrical interconnect connects the read/write head, located on the slider at the end of the head suspension over a disk, to disk drive circuits at the proximal end of the head suspension. The electrical interconnect carries electrical signals from the read/write head that are read from the disk across the head suspension to disk drive circuitry. It also carries electrical signals to be written to the disk from the disk drive circuitry across the head suspension to the read/write head.
An electrical interconnect can be created by an additive circuit laying process where layers are essentially added to an insulating base.
An insulating base is sometimes called an insulating or insulative substrate, film, or base layer. The purpose of the insulating base is typically to provide a supporting surface, with low electrical conductivity, upon which circuitry can be added. It is common to call the side of the insulating base upon which circuitry can be added the “top” of the insulating base, and the opposing side the “bottom.”
An insulating base can be a single insulating layer or multiple layers. An insulating base can have multiple layers of insulating and non-insulating layers as long as an insulating layer is on the top. A non-insulating layer is typically conductive. A conductive layer is sometimes called a conductor layer, conductive substrate, or metal substrate, and can include a thin metal foil or a supporting metal plate. Non-insulating layers within an insulating base may include a layer or multiple layers which support the top insulating layer. If a layer supports the top insulating layer, then the supporting layer is sometimes called a supporting substrate. For example, a supporting substrate can be part of a stainless steel load beam.
In an additive process, a relatively thin seed layer is typically added to the top of an insulating base. A seed layer is sometimes called a thin metal film. A seed layer is added to the insulating base by vacuum deposition, such as sputtering. Typically, the seed layer is deposited over the entire top side of the insulating base so as to form a continuous sheet. The seed layer can be a single stratum of material, or it can be multiple stratums of material formed sequentially. The top stratum of a seed layer is typically the same material as will be used for the conductor, discussed below.
On top of the seed layer is typically added a plating resist pattern. Plating resist material, which makes up the plating resist pattern, prevents metal from plating on portions of the seed layer which it covers and provides a vertical profile to which the metal will conform. The plating resist pattern can be applied in a reverse pattern with respect to the final wired circuit pattern. The thickness of the plating resist is chosen as at least the thickness of the metal to be plated.
“Applying” a layer can include any method which forms a patterned layer on a surface. Applying can include the multiple steps of spreading a photosensitive resin on a surface, exposing the resin to a pattern of light or other electromagnetic radiation, developing the pattern (or reverse pattern), washing away the undeveloped resin, and baking the resin. Applying can also include the use of a dry film resist.
The insulating base, seed layer, and plating resist pattern are then subjected to a plating process in which a conductor is plated. Such plating is sometimes called “conductor-plating.” A conductor is any material, such as copper metal, which is suitable for carrying a majority of current in an electrical interconnect. The plated conductor is formed on the exposed seed layer, in a reversal pattern with respect to the plating resist pattern on the seed layer. The pattern that the conductor conforms to is sometimes called a wiring pattern or circuit pattern.
The term, “plating,” is a general surface-covering technique in which a metal is deposited onto a conductive surface. The term most often refers to electroplating or electroless plating, both typically performed in a liquid bath.
The insulating substrate, seed layer, plating resist pattern, and plated conductor arrayed in a wired circuit pattern are sometimes collectively called an inchoate electrical interconnect. The inchoate electrical interconnect is considered a workpiece.
The term, “workpiece,” can refer to any starting piece or any product of an intermediate process. A workpiece includes material or parts of a workpiece that will remain in a final product or be removed in later processes. A workpiece can be rigid or non-rigid. A workpiece is usually self-cohesive, but can include evanescent matter that will easily fall off, drip off, evaporate, or sublimate, such as a liquid on a surface that has not yet evaporated.
After conductor-plating, an optional layer of protective metal can be plated over the conductor. This is sometimes called “protective-metal-plating.” Protective metal can be any metal suited to protect a conductor underneath, such as nickel or a nickel-phosphorous alloy. Nickel is a suitable substance because nickel exposed to air forms a nickel-oxide layer which can protect an underlying conductor, such as copper, from oxidizing. The plating of nickel in disk drive head suspensions is often in the form of electroless nickel plating. Electroless nickel plating is commonly done at very high temperature.
The plating resist pattern can then be stripped from the workpiece, and portions of the seed layer which had underlain the plating resist pattern can be etched away. The etching of the seed layer, which typically covers the entire top side of the insulating base, can electrically isolate various conductor features, such as different wire traces.
A covercoat is then applied as an insulator to protect the conductor and prevent accidental shorts. A covercoat is sometimes cured (imidized) at elevated temperatures, such as 250° C. or more. The covercoat is rarely applied to terminal pads or other areas which will remain exposed.
Surfaces of the conductor which are still exposed, such as on terminal pads, can be coated with various metals for corrosion resistance, scratch protection, bonding facilitation, and other desirable properties. These metal coatings are often plated in what are sometimes called secondary plating operations.
Before secondary plating operations commence, the exposed surfaces are usually cleaned in a pretreatment process and activated with a weak acid etch. Etching at this stage in a process is sometimes called “surface-etching.” If an optional protective metal was plated over the conductor, then it has probably started to oxidize. Nickel-oxide forms more quickly at elevated temperatures. The optional protective metal is peeled away or etched with a strong acid etch from the terminal pads in preparation for subsequent plating.
After pretreatment and surface-etching, the exposed conductor surfaces are plated with a first layer of protective metal. This metal can be the same type used for the optional protective layer. To improve bonding and contact characteristics, terminal pads are sometimes plated with a second layer of contact metal, such as gold. This is sometimes called “contact-metal-plating.” Contact metal can be any metal used to improve the connection characteristics of a terminal contact, such as but not limited to gold or silver. Gold is often used as a contact metal because it is corrosion resistant, electrically conductive, ductile, and relatively nontoxic. Nickel is often used as the protective metal between a copper conductor and a gold layer because a nickel layer can reduce the occurrence of copper diffusing into gold and discoloring the gold surface.
Hard drive reliability is a critical part of hard drive qualification, and it is more important to hard drive applications requiring severe environmental conditions and a longer lifetime. One of the key drive reliability tests in some applications is hard disk drive corrosion testing, where components are tested at 85° C. at 85% relative humidity for anywhere from 24 hours up to 504 hours.
Examples of the manufacturing steps outlined above are disclosed in U.S. Pat. No. 6,399,899 issued to Ohkawa et al. in FIGS. 5-8 and U.S. Pat. No. 7,129,418 issued to Aonuma et al. in FIGS. 2-3.
There is a need for an improved electrical interconnect for a disk drive head suspension. Specifically, there is a need for an electrical interconnect which can better resist corrosion, particularly around its terminal pads, and a method for producing such an electrical interconnect.